Usually measurement units are coordinated with the "locale" of your Firefox build. If you check the "Internationalization & Localization" section of the Troubleshooting Information page, do you have any unexpected locales listed there?

Ref. Use the Troubleshooting Information page to help fix Firefox issues

Normally, Firefox will try to use the default paper size set in the printer settings at the system level, but how that works is mysterious to me. Within Firefox, you could look at the following setting and see whether tweaking it manually helps at all:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. Since we have had this setting for a long time, I feel comfortable mentioning it.

(2) In the search box in the page, type or paste paper_id and pause while the list is filtered

Firefox should list the print.printer_yourprinter.print_paper_id preference for each of your printers. Ignore the Mozilla_Save_to_PDF printer for now.

(3) If you prefer Letter, double-click any preference whose value is not 1 and change the value to 1 and then press Enter or click the blue check mark button to save the change.

Does that stick or does Firefox override it the next time you call up print?

Thanks for the suggestions, no success so far.

Nothing surprising under "Internationalization and Localization" except possibly that en-CA is listed before en-US in Available Locales (don't know if order makes any difference there), but Requested Locales, Regional Preferences, and Default Locale are all en-US.

In about:config, there actually is no print.printer_Brother-MFC-5890CN.print_paper_id listed; only print.printer_Brother-MFC-5890CN.print_orientation and print.printer_Brother-MFC-5890CN.print_to_filename. I tried adding a print.printer_Brother-MFC-5890CN.print_paper_id with value of 1, but it had no effect. Tried giving it a value of 0 as well as a test, no effect either.

I'm not sure why all the other properties are missing. It sounds like Firefox is having trouble reading data from the system. However, I run Windows so there certainly could be some platform differences I don't know about.

You can try Firefox from the official Mozilla server if you currently use a version from the repositories of your Linux distribution (release-cck-mint) to see if it behaves differently.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

No difference with the downloaded version, unfortunately.